This application claims the benefit of Korean Patent Application No. 2000-35649, filed on Jun. 27, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a display device, and more particularly, to a multi-domain liquid crystal display (LCD) device and a method for fabricating the same.
2. Discussion of the Related Art
A Cathode Ray Tube (CRT) is one of display devices mainly used in monitors of information terminals and measuring instruments including telelvision. However, it is difficult for a CRT to be adapted to miniaturization and to have light weight due to its weight and size.
A liquid crystal display (LCD) device having a small size, light weight and low power consumption has been actively developed to substitute for such a CRT. Recently, the liquid crystal display devices can be configured as a flat panel display device. Thus, demand of the LCD device is increasing.
Such an LCD device is based on electric optical characteristic of a liquid crystal injected within a panel unlike a plasma display panel (PDP) or a field emission display (FED), the LCD device does not itself emit light. Accordingly, to view a picture displayed in an LCD, a separate light source, i.e., a back light assembly is required for uniformly irradiating light onto a display panel.
FIG. 1 shows a general LCD. Referring to FIG. 1, the LCD includes a first substrate 1, a second substrate la, and a liquid crystal (not represented) injected and sealed between the first and second substrates 1 and 1a. 
In more detail, on the first substrate 1, a color filter layer 2 is formed to display color, a black matrix layer 3 is formed to prevent light from being transmitted to a portion other than a pixel region of the second substrate, a common electrode 4 is formed to apply a common voltage Vcom to the panel.
On the second substrate 1a, a gate line 5 and a data line 6 are arranged to cross each other, so that a pixel region is formed in a matrix arrangement. A thin film transistor (TFT) and a pixel electrode are formed in each pixel region.
Currently, one of the most widely used liquid crystal displays is a twisted nematic (TN) mode LCD. The TN-mode LCD is constructed in a manner such that electrodes are formed on each of the two substrates, respectively. And liquid crystal molecules interposed between them are twisted in a spiral shape, parallel to the substrates and having a predetermined pitch.
In this structure, a voltage is applied to the electrodes to drive a liquid crystal director. However, the TN-mode LCD has poor contrast because light is not completely blocked in an OFF-state. Furthermore, it generates a gray inversion so that a contrast ratio varies with angle to invert luminance of medium gray, thereby causing difficulty in obtaining stabilized images. Moreover, the TN-mode LCD does not have satisfactory viewing angle characteristic.
A variety of research has been conducted in an attempt to solve the narrow viewing angle problem of the LCD. The research includes a film-compensated mode for compensating a viewing angle with a compensation film, a multi-domain mode in which pixels are divided into multiple domains and each domain has a different main viewing angle direction to compensate the viewing angle, an in-plane switching mode in which two electrodes are placed on the same substrate to generate a horizontal electric field, and an OCB (optically compensated birefringence) mode.
Meanwhile, a vertical alignment (VA) mode LCD uses a negative liquid crystal having a negative dielectric constant anisotropy and a vertical alignment film. In this type of the LCD, the longer sides of the liquid crystal molecules are arranged perpendicular to the plane of the alignment film when no voltage is applied, and a polarizing axis of a polarizer attached onto the substrate is perpendicular to the longer sides of the liquid crystal molecules, to produce a normally black mode.
On the other hand, when voltage is applied to the LCD, the longer sides of the molecules are moved from the direction perpendicular to the alignment film plane toward the alignment film plane to transmit the light according to the characteristic that the negative liquid crystal molecules, which are oriented and inclined with respect to the electric field.
The VA-mode LCD is superior to the TN-mode LCD in terms of contrast ratio, response time, and so on. Furthermore, in the case where a direction in which the liquid crystal molecules fall is divided into a predetermined number of multiple directions and a compensated film is employed, a viewing angle can be effectively realized.
Moreover, there have been recently proposed PVA (patterned vertical alignment) and MVA (multi-domain vertical alignment) in which structures such as side electrodes and ribs or slits are formed on the substrate to distort the electric field applied to the liquid crystal layer, instead of alignment treatment, thereby locating the liquid crystal molecular director in a desired direction.
FIGS. 2A to 2C are cross-sectional views for explaining problems of the TN LCD, while FIGS. 3A to 3C are cross-sectional views for explaining an alignment direction according to a rubbing process. Although the TN LCD among TFT LCDs has advantages of excellent contrast and satisfactory color reproducibility, it has a disadvantage of a narrow viewing angle.
Referring to FIG. 2A, in a normally white mode TN LCD, liquid crystal molecules 14 are aligned in the same direction with a slight inclination (about 1 to 5xc2x0) when no voltage is applied between two substrates 12 and 13 of the LCD. In this state, light is seen nearly white in any azimuth. In case of application of a voltage higher than a threshold value, as shown in FIG. 2C, intermediate liquid crystal molecules 14 except for those located near the substrates 12 and 13 are aligned in a vertical direction. Incident linearly polarized light is therefore blocked, but not twisted. At this time, light obliquely incident on a screen (panel) has the direction of polarization thereof twisted to some extent because it passes obliquely through the liquid crystal molecules 14 aligned in the vertical direction. The light is therefore seen halftone (gray), but not perfectly black.
As shown in FIG. 2B, in the state in which an intermediate voltage lower than the voltage applied in the state shown in FIG. 2C is applied, the liquid crystal molecules 114 near the alignment layers are aligned in a horizontal direction but the liquid crystal molecules 14 in the middle parts erect themselves halfway. The birefringent characteristic of the liquid crystal is lost to some extent. This causes transmittance to deteriorate and cause halftone (gray).
However, this refers only to the light incident perpendicularly on the liquid crystal panel. The obliquely incident light is seen differently, that is, light is seen differently depending on whether it is seen from the left or right side of the drawing. As illustrated, the liquid crystal molecules 14 are aligned mutually parallel relative to the light propagating from the light below to left above.
The liquid crystal hardly exerts a birefringence effect. Therefore, when the panel is seen from the left side, it is seen black. By contrast, the liquid crystal molecules 14 are aligned vertically relative to light propagating from light below to right above. The liquid crystal exerts a great birefringence effect relative to incident light, and the incident light is twisted. This results in nearly white display. Thus, the most critical drawback of the TN LCD is that a display state varies with a viewing angle.
It is known that a viewing angle of the liquid crystal display device (LCD) in the TN mode can be improved by setting orientation directions of the liquid crystal molecules inside pixels to a plurality of mutually different directions. Generally, the orientation direction of the liquid crystal molecules (pretilt angles) which keep in contact with a substrate surface in the TN mode are restricted by the direction of a rubbing treatment applied to the alignment film.
The rubbing treatment is a process in which the surface of the alignment film is rubbed in one direction by a cloth such as rayon, so that the liquid crystal molecules are orientated in the rubbing direction. Therefore, a viewing angle can be improved by making different rubbing directions inside the pixels.
FIGS. 3A to 3C show a method of making a different rubbing direction inside pixels. As shown in the drawing, an alignment film 22 is formed on a glass substrate 16. For simplicity, electrodes and other elements are omitted from the drawing. The alignment film 22 is then brought into contact with a rubbing roll 201 to perform a rubbing treatment in one direction.
Next, a photoresist is applied to the alignment film 22 and a predetermined pattern is exposed and developed by photolithography. As a result, a layer 202 of the photo-resist which is patterned is formed as shown in the drawing. Then, the alignment film 22 is brought into contact with the rubbing roll 201 that is rotating to the opposite direction to the above, so that only the open portions of the pattern are rubbed.
In this way, a plurality of regions that are subjected to the rubbing treatment in different directions are formed in the pixel, and there are a plurality of orientation directions of the liquid crystal inside the pixel. The rubbing treatment can be done in arbitrarily different directions when the alignment film 22 is rotated relative to the rubbing roll 201.
Although the rubbing treatment has gained a wide application, it is the treatment that rubs and consequently damages the surface of the alignment film. Also, in the rubbing treatment problems caused by dust are likely to occur. Another method of restricting a pretilt angle of the liquid crystal molecules in the TN mode involves a concave-convex pattern on an electrode. The liquid crystal molecules in the proximity of the electrodes are orientated along the surface having concave-convex pattern.
In addition, in the VA LCD, in which a vertical alignment and a negative liquid crystal are applied, the alignment direction of the liquid crystal molecules can be divided in order to improve viewing angle. In this case, it is preferable to increase the viewing angle corresponding to an in-plane switching (IPS) mode, while maintaining a contrast ratio and a response time as high as those of the conventional LCD.
It is possible to make domains in the VA mode uniform by arranging the liquid crystal molecules obliquely at the initial stage to be uniformly oriented in multiple directions in each pixel when an electric field is applied thereto. Here, the domain of at least one substrate must be divided, and an inclined surface should be formed on the substrate having the divided domains. The inclined surface includes one slanted nearly perpendicular to the substrate. The vertical alignment film is not required to be rubbed in this case.
The liquid crystal molecules are aligned perpendicular to the substrate when no voltage is applied to the VA LCD. However, they have a slope to the substrate due to the inclined surface. Upon application of the voltage, the liquid crystal molecules are tilted due to intensity of the electric field. At this time, the tilt angle due to the electric field has a rotation direction of 360xc2x0 because the electric field is created perpendicular to the substrate.
Recently, an LCD has been proposed, in which a liquid crystal is not aligned and is driven by auxiliary electrodes insulated from pixel electrodes.
FIG. 4 is sectional view of pixel unit of a related art LCD.
A related art LCD includes a first substrate and a second substrate, a plurality of data lines and gate lines arranged in first and second directions on the first substrate to divide the first substrate into a plurality of pixel regions, a thin film transistor (TFT) formed on each pixel region of the first substrate and composed of a gate electrode, a gate insulating film, a semiconductor layer, an ohmic contact layer and source/drain electrodes, a passivation film 37 formed over the whole first substrate, a pixel electrode 33 formed on the passivation film 37 to connect with a drain electrode, and an auxiliary electrode 21 formed on the gate insulating film to partially overlap the pixel electrode 33.
On the second substrate, a light shielding layer 25 is formed to shield the light leaked from the gate and data lines and the TFT, a color filter layer 23 is formed on the light shielding layer 25, a common electrode 17 is formed on the color filter layer 23, and a liquid crystal layer is formed between the first and second substrates.
An open area 27 of the common electrode 17 and the auxiliary electrode 21 formed to surround the pixel electrode 33 distort the electric field applied to the liquid crystal layer, so that liquid crystal molecules in a unit pixel are driven variously. This means that when a voltage is applied to the LCD, dielectric energy due to the distorted electric field arranges the liquid crystal directors in needed or desired positions.
In the related art LCD, however, the common electrode or the pixel electrode requires the open area to obtain a multi-domain effect. Accordingly, the manufacturing process of the LCD further includes a step of patterning the electrodes.
Also, if the electrodes do not have the open area or the width of the open area is narrow, the electric field distortion needed for domain division to divide the pixel region becomes weak. Accordingly, the time needed to stabilize the liquid crystal directors increases, and luminance is reduced due to disclination.
Accordingly, the present invention is directed to a multi-domain LCD device and a method for fabricating the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a multi-domain LCD device and a method for fabricating the same, in which an alignment direction of liquid crystal molecules is controlled to improve response time and picture quality.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a multi-domain LCD device according to the present invention includes: first and second substrates having pixel regions; a pixel electrode formed on the second substrate; a first side electrode formed along the periphery of the pixel electrode; a second side electrode formed in a diagonal direction of the pixel electrode; and first and second dielectric frames respectively formed in the same direction as the second side electrode on the first substrate corresponding to the second side electrode.
In another aspect, a method for fabricating a multi-domain LCD device according to the present invention includes the steps of: forming a first side electrode on a substrate in a matrix arrangement; forming a second side electrode to connect both ends with a corner portion of the first side electrode; forming a pixel electrode having a plurality of open regions at an upper side of the second side electrode; forming a color filter layer on an opposing substrate; forming a common electrode on the color filter layer; forming a first dielectric frame and a second dielectric frame on the common electrode to pass through a central portion of the first side electrode at both sides around the second side electrode; and forming a liquid crystal layer between the substrates.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.